Maladaptive Daydreaming: where wild minds come to rest
I'm not sure. There was once a theory that it was brought on by abuse, but that's since been disproved. That doesn't mean that's never the case, it just means that it's not always the case. I've been doing this all my life, but many have expressed starting later in life. I guess it's different for everyone. I think I was born this way, but that doesn't mean it's the case for everyone.
That's basically how I became an MDer, I simply daydreamed as a young child out of boredom and now here I am.